7.4 Conservation tillage and cover crops

Conservation tillage

Conservation tillage is an umbrella term that covers a range of reduced-tillage and no-till systems. A key aspect of conservation tillage systems is to leave sufficient crop residue in the field to substantially reduce erosion potential of rain and wind. No-till leaves all residue in place. Some crop residues decompose quickly and provide less protection than others. Under a no-till system, seeds are planted with a seed drill or using a plow attachment that cuts a shallow furrow through the crop residue but only very shallowly into the soil. Rates of adoption of no-till are variable in the US, with the areas of highest adoption above 40% (Fig 1). Where year-round growing seasons occur, conservation tillage may also be an important means of increasing soil nutrients.

Other conservation tillage systems employ some kind of soil-disturbing process, but limit the depth and width of the disturbed area, to leave as much undisturbed soil as possible. The soil is often only sliced rather than sliced and overturned. Strip tillage usually combines strips of reduced-depth tillage with untilled strips, to minimize soil disturbance and retain some of the benefits of the crop residues. These intermediate measures allow farmers to get some of the benefits of tillage while minimizing erosion and loss of beneficial soil organisms and soil characteristics.

Conservation tillage may require that farmers re-tool to handle planting, fertilizing, and pest management while protecting crop residues and minimizing soil disturbance. Old equipment cannot be modified for the new techniques. The expense of farm equipment means that such changes in farm practice take significant investment and result in sunk costs – investments that cannot easily be recovered in the event of a change of plans. Farmers are understandably reluctant to undertake such changes without clear evidence that the results will be worthwhile.

Conservation tillage has limitations. In wet or compacted soils, the limited disturbance leaves soils poorly prepared for planting. In cold and wet conditions, crop residues may harbor more disease organisms and harmful molds, leading to crop loss. Without the soil turnover of standard tillage, weed loads are much higher, requiring additional expense to eliminate. Long-term conservation tilling also risks compaction in subsoil from heavy machinery use to plant and harvest crops.^[1] In the future, light, autonomous (self-driving) farm equipment may reduce compaction.

Cover crops

Cover crops are crops grown after the primary crop is harvested, to protect the soil, reduce weeds, aid in carbon sequestration in the soil, and, in some cases, to add an additional crop. Some cover crops can be used to take up excess nutrients in the soil to reduce nutrients in field run-off and protect water quality. Other crops, particularly legume crops (clover, vetch, pea) can be used to add N to fields as natural fertilizers, to replace synthetic fertilizer. Cover crops are typically used used in areas with a cold or a dry season and must be able to tolerate the harsher conditions between crops. Rates of cover-crop adoption are still low in the US (Fig 2).

All cover crops protect soil, but their ability to reduce weeds depends on the nature of the problem weeds in the area. Some weeds, particularly perennial weeds, can compete well against cover crops. Some cover crops have an allelopathic effect – they are harmful to other plants. Usually, these effects work better against annual weeds than perennial weeds. And farmers must be careful not to choose cover crops or cover-crop schedules that may expose crops to allelopathic effects that could reduce growth and yield.

Cover crops protect water quality by limiting erosion, but they also take up excess nutrients from the soil. In a field that is left fallow during the non-growing season (the cold season or the dry season), any fertilizer residues or nutrients from decomposing crop residues can leach into nearby waters. Cover crops reduce the likelihood of leaching and reduce eutrophication.

After a harvest, mycorrhizal fungi lose their plant partners. A cover crop provides plant roots to continue to support the fungi. In one study, the soybean crop planted in the growing season after the cover crop grew larger and took up more P than the crop planted after the field was fallowed. ^[2]

When it is time to plant the new primary crop, cover crops must be killed, unless the primary crop is strong enough to grow through the cover crop and shade it out or outcompete it. Broad-spectrum herbicides are often used to kill cover crops and any weeds. Such herbicides may drift, if applied aerially, or run off into streams, and can kill native plants and harm beneficial insects and animals of other kinds. Crimping – using a heavy roller to flatten the cover crop – can be used to harm the crop mechanically, effectively killing it and reduce its ability to block light to seedlings (Fig 3). Crimping creates a mulch layer – a layer of organic material over the soil – that reduces weeds and protects soil moisture. It also helps to protect against erosion until the crop is established and can provide that protection.

Green manures are a variation of cover crops that are grown specifically to be tilled back into the soil before the commercial crop is sown. The tilled-in vegetation breaks down and provides nutrients for the new crop while also improving aeration and water storage. The green-manure crop can take up excess nutrients after the commercial harvest, turn those nutrients into plant material over the winter, which then decomposes after it’s tilled in. Legumes are common green manure crops.

Knowledge Check

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